The present invention generally relates to a novel class of ceramic materials and to methods for the preparation of self-supporting ceramic materials. The ceramic materials of this invention have a unique polycrystalline structure which is strong and fracture-tough, and preferably are formed upon oxidation of a molten metal precursor with a vapor-phase oxidant.
The method by which the ceramic materials of this invention are formed is based upon the discovery of conditions which produce a surprising oxidation behavior of a metallic material. An appropriate body of metal or alloy is melted within a particular temperature envelope in a vapor-phase oxidizing atmosphere, and the liquid or molten metal forms a polycrystalline oxidation reaction product upon contact with the oxidant. With continued exposure to the process conditions, molten metal is drawn through the polycrystalline material toward the oxidizing atmosphere by a wicking or a capillary force. This transported metal forms additional oxidation reaction product upon contact with the oxidant. As the process continues, additional metal is transported through the polycrystalline formation thereby continually forming or "growing" a ceramic structure of interconnected crystallites. The resulting ceramic material optionally contains dispersed or occluded metal or alloy, which may or may not be interconnected, and whose presence depends to a greater or lesser degree upon such factors as process conditions, as explained below.
The materials of this invention can be grown with substantially uniform properties throughout their cross-section to a thickness heretofore difficult to achieve by conventional processes for producing dense ceramic structures. The process which yields these materials also obviates the high costs associated with conventional ceramic production methods, including fine uniform powder preparation, green body forming, binder burnout, and densification by sintering, hot pressing and/or hot isostatic pressing. The products of the present invention are adaptable or fabricated for use as articles of commerce such as by machining, grinding, polishing, etc. which, as used herein, is intended to include, without limitation, industrial, structural and technical ceramic bodies for such applications where electrical, wear, thermal, structural or other features or properties are important or beneficial; and is not intended to include recycle or waste materials such as might be produced as unwanted by-products in the processing of molten metals. Such ceramic bodies include, for example, engine components, heat exchangers, kiln furniture, cutting tools, abrasives, valve components, pump components, bearings, seals, dies, diffusion tubes, mufflers, tiles, radiation barriers, etc.
Ceramics have, in recent years, been increasingly considered as candidates for structural applications historically served by metals. The impetus for this substitution has been the superior properties of ceramics, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities when compared with metals, coupled with the fact that the engineering limits of performance of many modern components and systems are now gated by these properties in conventionally employed materials.
However, the key to substituting ceramics for metals in such structural applications has been the cost-effective development of strength and fracture toughness characteristics sufficient to withstand tensile loading, vibration and impact. Current efforts at producing high strength, reliable monolithic ceramics have focused upon improved powder processing technologies, and although these efforts have resulted in improvements in ceramic performance, they are also complicated and generally less than cost-effective. The emphasis in such technologies has been in two areas: (1) improved methods of producing ultra-fine, uniform powder materials using sol-gel, plasma and laser techniques, and (2) improved methods of densification and compaction, including superior techniques for sintering, hot pressing and hot isostatic pressing. The object of these efforts is to produce dense, fine-grained, flaw-free microstructures and, in fact, some improvement in performance capabilities in ceramics has been attained in some areas. However, these developments tend to result in dramatic increases in the cost of producing ceramic structures.
One limitation in ceramic engineering which is aggravated by trends in modern ceramic processing is scaling versatility. Conventional processes aimed at densification (i.e., removal of voids between powder particles) are incompatible with large, one-piece structural application possibilities for ceramics such as monolithic furnace liners, pressure shells, boiler and superheater tubes, etc. Several practical problems are encountered in the conventional processing of ceramic parts with an increase in part size. The problems include, for example, increased process residence times, stringent requirements for uniform process conditions over a large process volume, cracking of parts due to non-uniform densification if conditions are not sufficiently uniform, excessive compaction forces and die dimensions if hot pressing is used, and excessive pressure vessel costs due to internal volume and wall thickness requirements in the case of hot isostatic pressing.
The present invention overcomes these difficulties by producing dense, high strength and fracture-tough ceramic microstructures by a mechanism which is more direct and less expensive than conventional approaches.
The present invention also provides means for reliably producing ceramic materials based on oxidation reaction products of a size and thickness which is virtually impossible to duplicate with the presently available technology.
As used in this specification and the appended claims "oxidation reaction product" means one or more metals in any oxidized state wherein a metal has given up electrons to or shared electrons with another element, compound, or combination thereof. Accordingly, an "oxidation reaction product" under this definition includes the product of the reaction of one or more metals with an oxidant such as oxygen, nitrogen, a halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium and compounds and combinations thereof, for example, methane, ethane, propane, acetylene, ethylene, propylene, and mixtures such as air, H.sub.2 /H.sub.2 O and CO/CO.sub.2, the latter two (i.e., H.sub.2 /H.sub.2 O and CO/CO.sub.2) being useful in reducing the oxygen activity of the environment. Although the present invention is hereinafter described with particular emphasis on aluminum and specific embodiments of aluminum as the parent metal, this reference is for exemplary purposes only, and it is to be understood that other metals such as zirconium, titanium, silicon, tin, etc. also can be employed which meet the criteria of the invention.
The process of this invention relies upon a surprising oxidation characteristic of metals, and, therefore, it may be useful to review briefly what is known about the general oxidation behavior of metals and the previous limited use of metal oxidation as a mechanism for generating ceramic bodies.
Metals oxidize generally in one of four modes.
Certain metals oxidize upon exposure to an oxidizing environment and form an oxide which flakes, spalls or is so porous that the metal surface is continually exposed to the oxidizing environment. In such a process, a free-standing oxide body is not formed but, instead, a mass of oxide flakes or particles is formed. Iron, for example, oxidizes in such a manner.
Certain other metals such as aluminum, magnesium, chromium, nickel or the like oxidize in such a manner as to form a relatively thin, protective oxide skin which transports either oxygen or metal at such a low rate that the underlying metal is effectively protected from further oxidation. This mechanism does not yield a free-standing oxide structure of any significant structural integrity.
Still other metals are known to form a solid or liquid oxide film which does not protect the underlying parent metal because such oxides permit the transport of oxygen. While an oxygen-permeable film may retard the oxidation rate of the underlying metal, the metal itself is not totally protected by the film due to oxygen-permeability. Silicon is an example of a metal which exhibits this type of oxidation behavior, and when exposed to air at elevated temperatures, forms a glassy skin of silicon dioxide which is permeable to oxygen. Typically, these processes do not occur at nearly fast enough rates to produce a useful thickness of ceramic oxide material.
There also are metals which, upon oxidation, volatilize oxide reaction product thereby continually exposing fresh metal to oxidation. Tungsten is an example of a metal which oxidizes in this manner.
In an attempt to produce ceramics of greater thickness, fluxes have been added to the surfaces of metals such as aluminum and magnesium to dissolve or break up their oxides and render them susceptible to oxygen or metal transport so that thicker oxide skins can be produced. However, the ability to form free-standing oxide structures by such a technique is still limited to thin sections of relatively limited strength.
The use of this technique with admixtures of metal powders and other particulates to afford intrinsically porous, low strength ceramics is described by H. Talsma in U.S. Pat. No. 3,255,027 and W. A. Hare in U.S. Pat. No. 3,299,002.
Similar methods for producing thin-walled aluminum oxide refractory structures are also described by D. R. Sowards in U.S. Pat. No. 3,473,987 and R. E. Oberlin in U.S. Pat. No. 3,473,938, and L. E. Seufert discloses in U.S. Pat. No. 3,298,842 an analogous method for producing thin-walled hollow refractory particles.
However, one disadvantage with these prior art processes is the limited thickness of oxide formed, apparently because the effect of the fluxing agent is of relatively short duration so that the oxide becomes slow-growing and assumes a protective character after only a limited amount of oxide growth. Increasing the flux concentration to promote thicker oxide growth is counterproductive because it results in a product which is lower in strength, refractoriness, and hardness.
One technique which has been successfully employed to create free-standing ceramics by the oxidation of metals involves an oxidation/reduction or "redox" type reaction. It has long been known that certain metals will reduce other metal oxides to form a new oxide and, also, a reduced form of the original oxide. The use of such redoxtype reactions to produce ceramic materials is described in U.S. Pat. No. 3,437,468 to L. E. Seufert and U.S. Pat. No. 3,973,977 to L. E. Wilson. An important disadvantage of the redox-type reactions described in these patents is their inability to produce a singular, hard, refractory oxide phase as a reaction product, but rather the products contain multiple oxide phases which degrade the hardness, modulus of rupture and wear resistance characteristics.
The present invention involves a unique and novel oxidation phenomenon which differs from any of the classical oxidation modes and which overcomes the difficulties and limitations associated with known processes.
The invention will now be described by reference to the Drawings.